Referring to Prior Art FIG. 1, a typical system for balancing cells in the battery 12, in accordance with the prior art is illustrated. This system includes a relay 14 connected to a resistor 23 for each cell 21, and a voltage monitoring circuit 25 for the battery 12. The voltage monitoring circuit 25 monitors the cell voltage across each cell 21. Relay 14 will be switched on to discharge the corresponding cell 21, if the cell 21 has a higher cell voltage than any other cell of the battery 12. Relay 14 will be switched off once the cell 21 is adequately balanced. However, the relay dissipates much power and its switching on-off speed is slow. In addition, since relay 14 is comparatively large, it holds more PCB space.
An alternative is to use power switches rather than relays to balance cells. However, this solution in the prior art needs more complex high-voltage control circuits. As a result, larger die size is required, which does not work well for multiple cells. In addition, this solution in the prior art has a fixed control scheme, which is not capable of flexible cell balance control.
With reference to Prior Art FIG. 2, another system for balancing cell voltages of a Lithium-Ion battery pack 10 is shown. For each battery cell 10-1, 10-2 and 10-3, a cell balance circuit includes a resistor 20, a transistor 22, and a control circuit 24. The resistor 20 and the transistor 22 are connected in series, with their end terminals connected to two terminals of the battery cell 10-1, 10-2, or 10-3 respectively. Each control circuit 24 controls the conducting state of the transistor 22, such that two terminals of the battery cell 10-1, 10-2, or 10-3 are short-circuited through the transistor 22 as controlled by the control circuit 24.
Prior Art FIG. 3 shows the detailed circuit configuration of the control circuit 24. The control circuit 24 includes an oscillation circuit 30, a comparator 26, two input terminals 28 and 32 connected to the positive and negative terminals of the corresponding battery cell 10-1, 10-2, or 10-3 respectively, and an output terminal 34 connected to the gate of the corresponding transistor 22.
The oscillator 30 generates a saw-tooth wave voltage which oscillates within a predetermined voltage range. The oscillator 30 will output a voltage equal to the summation of the saw-tooth wave voltage and the voltage at terminal 32 (negative terminal of the cell 10-1, 10-2, or 10-3). Comparator 26 will compare the voltage at the terminal 28 (positive terminal of the cell 10-1, 10-2, or 10-3) with the output voltage from the oscillator 30, and output a comparative signal indicating whether the voltage across the battery cell 10-1, 10-2, or 10-3 exceeds the predetermined voltage range of the oscillator 30. When the voltage over the battery cell 10-1, 10-2, or 10-3 exceeds the predetermined voltage range in the oscillator 30, the control output from the comparator 26, which is a PWM (pulse-width-modulated) signal, will switch on the corresponding transistor 22 to allow the bypass current.
However, the balancing scheme of Prior Art FIG. 3 is fixed by the predetermined voltage range which is set in the oscillator. As result, this conventional method lacks the flexibility to adjust the predetermined voltage range for different kinds of batteries. Furthermore, expensive high-accuracy comparators are required for good cell balancing in Prior Art FIG. 3.